Do not ask me how it happened. But I lost my password .
Strange, but after a system upgrade my usual password did not work more.
Panic !!!
The solution is here....
(I thought it was safe linux)
1) you have to reboot into recovery mode.
If you have a single-boot (Ubuntu is the only operating system on your computer), to get the boot menu to show, you have to hold down the Shift key during bootup.
If you have a dual-boot (Ubuntu is installed next to Windows, another Linux operating system, or Mac OS X; and you choose at boot time which operating system to boot into), the boot menu should appear without the need to hold down the Shift key.
From the boot menu, select recovery mode, which is usually the second boot option.
Select root / Drop to root Shell prompt
2) run following:
mount -rw -o remount /
passwd root (where root is the name of the user that you want change the password)
exit
3) select resume
